Heretofore, a mobile communication system has been known which can include a radio access network (UTRAN: Universal Terrestrial Radio Access Network, or GERAN: GSM EDGE Radio Access Network) of a 2G/3G system and a radio access network (EUTRAN: Evolved Universal Terrestrial Radio Access Network) of an LTE (Long Term Evolution) system.
3GPP TS 23.216 defines “SRVCC (Single Radio Voice Call Continuity) system” for switching a path for voice communications between a UE (User Equipment) #1 and a UE#2 from a path (path for VoIP (Voice over IP) communications) through E-UTRAN or UTRAN to a path (path for circuit switch (CS) communications) through UTRAN/GERAN.
In addition, Alt. 4, Alt. 11, and the like of 3GPP TR 23.856 v0.4.1 define an SRVCC system (hereinafter referred to as enhanced SRVCC system) for switching the path for voice communications between the UE#1 and the UE#2 from the path through E-UTRAN to the path through UTRAN/GERAN with an MGW (Media gateway) #1 in a visited network of the UE#1 used as an anchor point.
With the enhanced SRVCC system, a signal required for switching the path for voice communications between the UE#1 and the UE#2 can be terminated within the visited network of the UE#1. Thus, instantaneous communication-interruption time due to the exchange of the signals between the visited network of the UE#1 and the home network of the UE#1 can be shortened from that in the conventional SRVCC system.
A procedure for establishing a path for voice communications between the UE#1 and the UE#2 is described below with reference to FIG. 7.
As shown in FIG. 7, the UE#1 sends “INVITE” to a P-CSCF (Proxy-Call Session Control Function)/VATF (Visited Access Transfer Function) in Step 1.
In Step 2, the P-CSCF/VATF allocates the MGW#1 to the path.
The P-CSCF/VATF sends “INVITE” to an I/S-CSCF (Interogation/Serving-Call Session Control Function), “INVITE” including identification information on MGW#1 and “eSRVCC” indicating that an enhanced SRVCC system can be used.
In Step 3, the I/S-CSCF sends “INVITE” including “eSRVCC” to SCC AS (Service Centralization and Continuity Application Server).
In Step 4, the SCC AS sends “INVITE” to the I/S-CSCF. In Step 5, the I/S-CSCF sends “INVITE” to the UE#2.
In Step 6, the UE#2 sends “200 OK (response signal to “INVITE”)” to the I/S-CSCF. In Step 7, the I/S-CSCF sends “200 OK” to the SCC AS. In Step 8, the SCC AS sends “200 OK” to the I/S-CSCF. In Step 9, the I/S-CSCF sends “200 OK” to the P-CSCF/VATF. In Step 10, the P-CSCF/VATF sends “200 OK” to the UE#1.
As a result, a signaling path for voice communications between the UE#1 and the UE#2 is established among the UE#1, the P-CSCF/VATF, the I/S-CSCF, and the UE#2, and a media signal path for voice communications between the UE#1 and the UE#2 is established among the UE#1, the MGW#1, and the UE#2.